This invention relates to soybean plants possessing a novel resistance to Phytophthora sojae, which maps to a chromosomal locus and methods for identifying and breeding these plants, the methods involving marker assisted selection.
Soybeans, (Glycine max L. Merr) are a major cash crop and investment commodity in North America and elsewhere. Soybean oil is one of the most widely used edible oils, and soybeans are used worldwide both in animal feed and in human food production. Phytophthora root and stem rot is a devastating disease of soybean that occurs throughout the U.S. and the world (Wrather et al., Can. J. Plant Pathol., 2001). Phytophthora root and stem rot caused by Phytophthora sojae is the second leading cause of yield loss in soybean in the United States. Yield losses in 1998, due to Phytophthora root and stem rot in top soybean producing countries were 1149 and 92 thousand metric tons in the U.S. and Argentina, respectively (Wrather et al., Can. J. Plant Pathol., 2001). General resistance mechanisms against P. sojae include structural features of the host, preformed chemical inhibitors, induced structural barriers, hypersensitive reactions and phytoalexins (Erwin et al., eds, Phytophthora, 1983). Phytophthora root and stem rot was first described in Ohio and shortly thereafter it was described in Indiana and North Carolina (Suhovecky and Schmitthenner, 1955). The pathogen is now referred to as Phytophthora sojae. 
Resistance to Phytophthora root and stem rot is a trait provided by multiple alleles. To date, thirteen resistance (Rps) alleles at seven loci have been described; Rps1 (Bernard et al., Agron. J., 1957), Rps2 (Kilen et al., Crop Sci., 1974), Rps3 (Mueller, Phytopathology, 1978), Rps4 (Athow et al., Phytopathology, 1980), Rps5 (Buzzell and Anderson, Soybean Genet. Newslett., 1981), Rps6 (Athow and Laviolette, Phytopathology, 1982), and Rps7 (Anderson and Buzzel, Plant Dis., 1992). The Rps resistance loci are found on soybean major linkage groups (MLGs) N, J, F, and G (Demirbas et al., Crop Science, 2001; Diers et al., Crop Science, 1992). Populations of P. sojae exist in many soybean production regions that cause disease on plants with many, if not all, of these Rps genes.
Single gene resistance has provided adequate disease management; however, each single gene deployed in a soybean cultivar is only effective for eight to fifteen years (Schmitthenner, Plant Dis., 1985). Pathotypes of P. sojae, containing virulence genes to Rps1k (the most recently deployed Rps gene) have already been found in fields throughout the midwest (Abney et al., 1997; Dorrance et al., 2003, Kaitany et al., 2001; Kurle and ElAraby, 2001; Leitz et al, 2001; Schmitthenner et al., 1994; Tang et al., 1996). Accordingly, novel resistance loci or alleles are desirable for introduction into commercial soybean lines to protect against yield losses caused by P. sojae. 